GAZA CITY, Gaza Strip — A rally of more than 250,000 Fatah supporters ended in mayhem Monday, with Hamas police opening fire and demonstrators hurling rocks or running for cover.

Seven civilians were killed and dozens of people were wounded in the intra-Palestinian violence.

The rally in a Gaza City square, marking the Nov. 11, 2004 death of Palestinian leader Yasser Arafat, posed the strongest challenge to Hamas rule in Gaza since the Islamic militant group seized the impoverished territory by force in June.

Fatah’s unexpected show of strength was also its most persuasive argument that its leader, moderate Palestinian President Mahmoud Abbas, has a mandate to renew peacemaking with Israel at a U.S.-hosted Mideast conference in Maryland later this month.

The march started out peacefully after Palestinians in buses, on foot and on donkey carts streamed into a large sandy lot by the Mediterranean. Waving yellow Fatah flags, the crowd cheered and whistled as the movement’s remaining leaders in Gaza — most had fled to the West Bank in June — delivered hopeful speeches about a comeback.

But the rally quickly descended into chaos and bursts of gunfire as the crowd was beginning to leave. Fatah stone-throwers clashed with armed Hamas policemen.

“It was a big mess, and everyone was running for cover,” said a wounded 24-year-old demonstrator.

In one confrontation, about 30 Hamas policemen — some posted on the rooftop of a government ministry, others on the streets below — firing heavily for about 10 minutes. Some appeared to be shooting over the heads of Fatah stone-throwers while others were firing randomly.

The Fatah supporters pelted Hamas troops with stones, surging forward even as they were met by gunfire.

Elsewhere, frantic demonstrators commandeered private cars to take the wounded to hospitals. Medical officials said seven civilians were killed and 85 people were wounded.

Hamas spokesman Sami Abu Zuhri held Fatah responsible for the violence, saying Fatah gunmen had been posted on high-rise buildings overlooking the rally to shoot at Hamas police officers as a planned provocation.

Hamas said five of its policemen were wounded by sniper fire from the roof of the pro-Fatah Al Azhar University near the rally.

Fatah denied the charge, saying no armed men were allowed at the rally. An AP reporter saw 10 Fatah gunmen being turned away at the start of the rally. Medical officials said none of the dead was a Hamas policeman.

Tens of thousands of Fatah supporters still draw a monthly salary from Abbas’ West Bank government, ensuring loyalty under Hamas rule.

In a gesture of support for Abbas, Prime Minister Ehud Olmert disclosed plans Monday to release more than 400 Palestinian prisoners before the Maryland peace conference. Israel is holding nearly 9,000 Palestinians, and Palestinians have asked for 2,000 to be freed before the meeting.

Hamas’ takeover of Gaza has given momentum to peacemaking after Abbas threw the Islamic group out of government and installed a Western-backed administration.

This gave Abbas freedom to resume peace talks, and the international community has rallied behind him. The U.S.-sponsored conference is a key part of that strategy, aiming to relaunch peace talks that broke down in 2001.

Abbas, whose Gaza security forces suffered an embarrassing defeat by Hamas in just five days of fighting in June, said the Islamic militants committed a “heinous crime” Monday. He ordered three days of mourning for the dead and wounded, with Palestinian flags to fly at half-staff.

The rally marked the third day of memorials for Arafat, who in death remains the most popular Palestinian politician; even Hamas has tried to co-opt him, portraying him as a leader who stood up to Israel.

In an echo of the rally’s violence, clashes erupted later Monday during the funeral for one of the victims, Ayoub Abu Samra, 19, who witnesses said had been shot and killed after getting into a scuffle with a Hamas policeman.

Mourners fired in the air and said Hamas police fired at the procession. Hamas police denied opening fire and said the marchers threw stones at them.

Rescue workers said three people were hurt, one by gunfire and two by beatings.